That viral 'diet drinks cause dementia' story reveals a bigger problem with how science gets reported

Several stories linking diet soda and sugary drinks with dementia have gone viral.

While they're based on two well-done scientific studies, they don't tell you the full story.

Before you clean out your fridge, you should know that the studies found a link between the two things — they did not find that one thing causes the other.

Read on to find out whether diet or regular is better for your health.

If you're reading this, chances are you've heard one or two scary-sounding stories about sweet drinks and brain health.

Either you heard that artificially-sweetened diet drinks cause dementia or that conventionally-sweetened ones shrink your brain. It all comes down to two studies published in two different reputable health journals.

At this point, you might be ready to throw up your hands and decide that all you can safely drink is water. Not so fast.

The problem with observing

If you've ever watched the show "Fringe," you're familiar with a group of characters called The Observers. (If you haven't seen the show, no worries. This will all make sense soon.) The Observers know what's going to happen in the future and what's happened in the past. So, even if they are watching something tragic unfold in real time — like a child dying, for example — they are powerless to change it. They can only watch and observe.

An Observer watching a scene unfold in Fringe. Netflix Both of the studies on sweet drinks and brain health fall into a category of research known as observational studies. Observational studies can tell us if there's a connection between two things— such as smoking marijuana and having anxiety — but they can't tell us if one thing necessarily causes the other.

In many cases, a link that researchers observe between two things is later found to be caused by an external thing that no one was accounting for. In the marijuana example, some researchers have suggested that people who deal with anxiety may simply be more likely to use cannabis than people who don't. So instead of cannabis causing anxiety, what's really going on is that people with anxiety are using more cannabis than people without it. (The science is still out on this one, in case you were wondering.)

For the sweet drinks and brain health research, the scientists drew from a large set of observational data taken from thousands of people from the town of Framingham, Massachusetts who were initially recruited beginning back in the 1940s as part of a study designed to learn more about heart disease called the Framingham Heart Study. Over the past few decades and through multiple generations of people, the researchers have kept tabs on the health of these individuals.

So, for that first study on sugary drinks, the scientists zeroed in on the Framingham community's penchant for soda and juice. They found that on average, the more sweet beverages people drank, the lower their total brain volume and the lower their scores on memory tests. Importantly, brain shrinkage has been tied to an increased risk of Alzheimer's disease.

For the second study, the researchers looked at the same data again, but this time focused on diet drinks instead. They found a worrisome link here too: People who regularly drank artificially-sweetened sodas (we're talking about 1-2 diet sodas per day) had a higher risk of stroke and dementia than people who didn't.

Neither of these findings are good. And the fact that the research is based on large groups of people lends some additional oomph to their conclusions. But the most important takeaway here is that there is simply no definitive research that tells us that sugary drinks cause brain shrinkage or that diet drinks cause dementia. Plus, out of all the people in the study, the percentage of those who did go on to develop stroke or dementia was small— about 3% for stroke and about 5% for dementia.

So the next time you're at a restaurant and are faced with the question, "Diet or regular?", what should you choose?

Diet or regular?

One thing the two current studies on sweet drinks simply can't tell us is which one is better for your brain.

La Piazza Pizzeria/FlickrPase, the lead author on both papers, says holding out on both is the safest option. "We recommend that people drink water on a regular basis instead of sugary or artificially sweetened beverages," he says in a press release.

Specifically, the researchers found "strong evidence for the independent role of the intake of sugar-sweetened beverages, particularly soda, in the promotion of weight gain and obesity in children and adolescents," they write.

Another recent paper written by seven experts in public health, nutrition, and economics makes the links between sugary drinks and America's obesity problem explicit:

"The science base linking the consumption of sugar-sweetened beverages to the risk of chronic diseases is clear," the authors write.

Above all else, everything is best in moderation. If you're going to drink either plain old sweet tea or soda, limit yourself to an 8-oz glass every few days, and have it as a snack rather than an accompaniment to one. Similarly, if you're going to drink diet soda, try and have it once a week rather than every day.